feat(ingestion): implement brain_query MCP tool

Wraps the existing search.Query function. Same BM25 over
brain/knowledge/ and brain/wiki/ that the HTTP /query endpoint serves.
Plan note: handleCall switch replaces the single-line stub from Task 1
— no unknownToolError type to remove since Task 1 inlined the error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/handlers_test.go

@@ -0,0 +1,52 @@
+package mcp_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func toolCall(t *testing.T, srv http.Handler, name string, args map[string]any) map[string]any {
+	t.Helper()
+	bodyBytes, err := json.Marshal(map[string]any{
+		"jsonrpc": "2.0", "id": 1, "method": "tools/call",
+		"params": map[string]any{"name": name, "arguments": args},
+	})
+	require.NoError(t, err)
+	req := httptest.NewRequest(http.MethodPost, "/mcp", bytes.NewReader(bodyBytes))
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+	require.Equal(t, http.StatusOK, rr.Code)
+	var resp map[string]any
+	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
+	return resp
+}
+
+func TestBrainQueryReturnsResults(t *testing.T) {
+	brainDir := t.TempDir()
+	knowledge := filepath.Join(brainDir, "knowledge")
+	require.NoError(t, os.MkdirAll(knowledge, 0o755))
+	require.NoError(t, os.WriteFile(
+		filepath.Join(knowledge, "tdd.md"),
+		[]byte("# TDD\n\nTest-driven development is iterative.\n"),
+		0o644,
+	))
+
+	srv := mcp.NewServer(brainDir, nil, nil)
+	resp := toolCall(t, srv, "brain_query", map[string]any{"query": "tdd"})
+
+	require.Nil(t, resp["error"])
+	result := resp["result"].(map[string]any)
+	content := result["content"].([]any)
+	require.NotEmpty(t, content)
+	text := content[0].(map[string]any)["text"].(string)
+	assert.Contains(t, text, "tdd.md")
+}